Following is what I (think to) know about Kevlar coming from kayak racing:
Kevlar is lighter and stronger than carbon. Kevlar fibre of a nominal length and diameter is lighter and able to hold more weight than a carbon fibre of the same parameters. However, Kevlar is way more flexible than carbon and kayak hulls built from Kevlar only have unsufficient rigidity assuming the same minimal amount of resin (the less resin, the better, lighter boat) . The result is that kayak hulls are buit with mixture of kevlar and carbon fibres. The ratios vary based on the purpose of the boat. Sprinters need to be super rigid (built mostly of carbon), marathon boats don't need that much rigidity, light weight and resistance to some abuse are desired, hence Kevlar is the material of choice. In the real world it is probably always a mixture of both types of fibres with every manufacturer swearing by its own formula.
How does this translate in bikes and bike wheels I am not too sure. My feeling is that Kevlar would make them too flexible.
That's my $0.02.
